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This study investigates the variations in the microstructure and mechanical properties of a medium-carbon high-Si steel (Fe–0.55C–2.3Si) during graphitization heat treatment and subsequent quenching and tempering heat treatment. The microstructure comprising ferrite and pearlite of the as-rolled sample changes to a more ductile microstructure with fine graphites uniformly dispersed in the ferrite matrix after graphitization treatment. These formed graphites are completely dissolved into the ferrite matrix after austenitizing treatment at 900 °C, but preexisting graphite positions remains as voids. The average size and number density of voids are nearly identical to those of the pre-existed graphites. The subsequent tempering treatments at 300 °C and 500 °C cause little changes in the size and number of the voids. Therefore, the formation of voids in the quenched-and-tempered samples is directly related to the graphites of the initial graphitized sample. The average size of graphites and voids is inversely proportional to the logarithm of their number density. When the as-rolled sample is completely graphitized, its ultimate tensile strength decreases from 928 to 494 MPa and its tensile elongation increases from 20.7 to 34.0%, indicating that the medium-carbon high-Si steel is substantially softened through graphitization treatment. Moreover, subsequent quenching and tempering treatment at 500 °C increases its ultimate tensile strength to 1168 MPa. The drastic softening through graphitization and significant strengthening through subsequent quenching and tempering exhibit the possibility of the application of medium-carbon, high-Si steels as cold heading quality steels for manufacturing fastener components.

The graphitization behavior of medium-carbon, high-silicon steel (Fe-0.55C-2.3Si) is investigated with particular focus on the effects of heat-treatment temperature and initial grain size on the graphitization rate and mechanical properties. With the progress of heat treatment, pearlite gradually disappears because of the decomposition of the cementite layers and graphites are formed because of the diffusion and aggregation of carbon atoms; these occurrences lead to a gradual decrease in the hardness of the material. A higher heat-treatment temperature promotes the graphitization behavior, which consequently leads to a decrease in the time taken for completion of graphitization and an increase in the average graphite size. Graphites are dominantly formed at grain boundaries owing to the high energy and disordered atomic arrangements at these boundaries. Further, because grain boundaries act as graphite nucleation sites, a smaller grain size corresponds to faster graphitization as well as the formation of finer and more abundant graphites. The complete graphitization treatment causes a significant reduction in the tensile and compressive strengths of the material and a considerable improvement in its ductility and cold forgeability.

Tempering approach is designed for better understanding the effects of heat treatment induced by production process when manufacturing on the transformation-induced plasticity steels containing 1.0 wt.% Al. Specific attention is placed on the roles of tempering temperature and the holding time on the decomposition of retained austenite and the redistribution of alloying elements. Using transmission electron microscopy, we found the retained austenite was decomposed into ε-carbide and ferrite in the steels tempered at 300 °C for 9 h. An increase in the temperature of 400 °C and the holding time over 3 h accelerate the nucleation kinetics of cementite formation, leading to the deteriorated thermal stability of austenite. In addition, atom probe tomography studies confirmed the partitioning tendency of alloying elements across the ferrite/cementite interfaces as well as the compositional spikes of Mn at the interfaces during tempering over 400 °C for 9 h. Keywords: Transformation-induced plasticity steels, Retained austenite, Decomposition, Atom probe tomography

In the present study, the effects of microstructure, chemical composition on the transformation behavior and mechanical stability of individual retained austenite (RA) with the different sizes and morphology in TRIP steels were intensively investigated. In order to characterize the property of the individual RA, various analytical techniques including atom probe tomography (APT), step-wise straining EBSD and nano-indentation were applied. The blocky type RA as we categorized have many defects and lower carbon contents (~25%) compared to film type RA. And also, step-wise straining EBSD and nano-indentation results revealed that the mechanical stability of blocky type RA was lower than that of film type RA which means blocky type RA would be easily transformed into martensite. It is considered that many defects existing in blocky type RA could work as the nucleation site of martensite transformation. In contrast, high carbon contents and hard phases enclosing film type RA would increase the resistance to shear transformation resulting in the inhibition of transformation of RA to martensite.

Transformation-induced plasticity (TRIP) steels have excellent strain hardening exponents and resistibility against tensile necking using the strain-induced martensite formation that occurs as a result of the plastic deformation and strain on the retained austenite phase. Detailed studies on the microstructures and local mechanical properties, as well as global mechanical properties, are necessary in order to thoroughly understand the properties of TRIP steels with multiple phases of ferrite, bainite, retained austenite, and martensite. However, methods for investigating the local properties of the various phases of the TRIP steel are limited due to the very complicated and fine microstructures present in TRIP steel. In this study, the experimental and numerical methods, i.e., the experimental nanoindenting results and the theoretical finite element analyses, were combined in order to extract the local stress–strain curves of each phase. The local stress–strain curves were in good agreement with the values presented in the literature. In particular, the global plastic stress–strain behavior of the TRIP steel was predicted using the multiple phase unit cell finite element analysis, and this demonstrated the validity of the obtained properties of each local phase. The method of extracting the local stress–strain curves from the nanoindenting curves and predicting the global stress–strain behavior assists in clarifying the smart design of multi-phase steels.

In the present study, the influence of tempering temperature on the microstructural evolution and prior austenite grain boundary segregation of AISI 4340 steels was investigated by transmission electron microscope and atom probe. The transmission electron microscopy results showed a variation in the microstructure and the morphology of carbides with a change in tempering temperature. Additionally, the chemical compositions of the prior austenite grain boundaries and carbides were quantified by atom probe tomography. An increase in the tempering temperature led to a decrease in the amount of carbon segregated at the prior austenite grain boundary from 7.9 to 1.3 at.%. It was found that a higher tempering temperature can accelerate the diffusion of carbon from the prior austenite grain boundary into carbide. However, phosphorus atoms were segregated mainly at the prior austenite grain boundary in steel tempered at 400°C (up to 0.18 at.%). It was found that formation of film-like carbide and phosphorus segregation along the prior austenite grain boundary is the main cause of embrittlement in steel tempered at 400°C.

In the present investigation the microstructure and age-hardening behaviour of Al–Mg–Si alloy prepared by twin roll casting (TRC), varying rolling speed (i.e., 3, 4, and 5 rpm), were studied. The as-cast samples were subjected to optical microscopy (OM) to understand the effect of rolling speed on the alloy microstructure. Significant difference in grain size and shape was observed for all the alloys in as-cast condition. The as-cast samples were solutionized at 540 °C for 2 h followed by isothermal heating at 180 °C for different time intervals. Thereafter, the as-cast and solutionized samples were subjected to scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). Segregation of solute atoms at grain boundaries were observed for both as-cast as well as solutionized samples. Age-hardening results show that time to attain peak hardness decreases for the alloy produced by higher rolling speed.

The grain boundary segregation of boron atoms in high strength low alloy steels containing 50 ppm boron was accomplished using atom probe tomography (APT) and nano-beam secondary ion mass spectroscopy (SIMS). The formation of boro-carbides under an excessive addition of boron to the steels was identified through the SIMS and TEM. The APT was performed in order to evaluate the composition of the alloying elements, such as, boron and carbon, segregated at prior austenite grain boundaries. The boron contents at the prior austenite grain boundaries were approximately 1.7 ± 0.2 at.%, which was approximately 70 times more than the amount of boron added to the steels.

In the present investigation the Al–5.5Mg–0.02Ti alloy produced by twin roll casting (TRC) process (varying rolling speed, i.e., 3, 4, and 5 rpm) has been subjected to heat treatment for microstructure modification. Grain coarsening at the center of the strip has been observed during heat treatment process. Homogeneous microstructure of the alloys has been achieved by heat treatment process, and it has been found that the time to achieve homogeneous structure depends on the rolling speed. Transmission electron microscope (TEM) studies revealed that undesired Mg rich phase (Mg5Al8) has been successfully eliminated by heat treatment process. Fine and equi-axed grains in the alloys obtained by heat treatment process shows high strength and elongation.

The medium carbon (0.5 wt% C) steels containing various boron contents were studied to observe the distribution of boron using atom probe tomography and electron energy loss spectroscopy. APT revealed the segregation of boron atoms at retained austenite for 100 ppm boron added steels and the trapped carbon atoms at micro-twins for 50 ppm boron treated steels. Moreover, it was also found that boron was randomly distributed for 20 ppm boron added steels regardless of the interactions between carbon and boron.

In the present investigation the microstructures and mechanical properties of Al–Mg alloys prepared by twin roll casting (TRC), varying rolling speed, (i.e., 3, 4, and 5 rpm) were studied. Optical microstructures of the all the samples show dendritic morphology. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS) studies show segregation of alloying elements at the interdendritic regions. Transmission electron microscopy (TEM) observation confirms the presence of intermetallic phase (Mg5Al8) at the grain boundaries. It was observed that the secondary dendritic arm spacing (SDAS) increases (marginally) with rolling speed. Also the microhardness and tensile strength decreases with increase in rolling speed.

An new method is proposed to predict in sequence the phase transformation behavior and the depth of the decarburized ferrite layer with simultaneous scaling of a steel during cooling process from the measured temperature history. This proposed method successively couples a set of mathematical models describing the temperature evolution, phase transformation, decarburization and oxidation. The dilatometry experiment was performed to verify the proposed method. Results reveal that the measured depth of the decarburized ferrite layer agreed well with the predicted depth. The predicted pearlite volume fraction nearly coincided with the measured value as well.